Age Determination of Cod from Otoliths Collected in Divisions 2J, 3K and 3L

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NAFO Sci. Cun. Studies, NO.1: -101 Age Determinatin f Cd frm Otliths Cllected in Divisins J, K and L R. Wells Department f Fisheries and Oceans, Research and Resurce Services P. O. Bx, St.

1 NAFO Sci. Cun. Studies, NO.1: -101 Age Determinatin f Cd frm Otliths Cllected in Divisins J, K and L R. Wells Department f Fisheries and Oceans, Research and Resurce Services P. O. Bx, St. Jhn's, Newfundland, Canada A1C X1 Intrductin Except fr labratry-reared specimens, it is perhaps impssible t determine with certainty the age in years f individual cd, Gadus mrhua, particularly thse cmprising the adult ppulatin. Cd taken in the cmmercial fisheries in the Nrthwest Atlantic are usually age and lder, and the catches may cnsist f 10 r mre age grups as are evident frm reprted age-length keys (e.g. ICNAF, 19). As cd becme lder and the average length-at-age tends t a maximum, the verlap in the length range f adjacent agegrups becmes mre prnunced. The prprtin f cd f a particular age at a given length varies because f such factrs as changes in grwth rate and fluctuatins in year-class strength. Therefre, the lengths f specimens taken in the cmmercial fisheries cant prvide reliable indicatins f age. Ages f cd have been determined frm varius skeletal elements, but interpretatin f age frm tliths is the usual prcedure. Cmparisn f length and age frequencies, reprted by varius labratries as being representative f cd catches frm particular stcks, indicated cnsiderable variatin in age determinatin f cd frm tliths by ageing experts f these labratries. In an attempt t reslve these discrepancies and t prvide guidelines fr interpretatin f cd tliths, the Standing Cmmittee n Research and Statistics f ICNAF (Internatinal Cmmissin fr the Nrthwest Atlantic Fisheries) rganized wrkshps at Vig, Spain, in Octber 19 (Lpez-Veiga et al., 19) and at St. Jhn's, Newfundland, Canada, in Feburary 19 (ICNAF, MS 19). Frm the wrkshp held at St. Jhn's, phtgraphs were available f tlith sectins fr which the ages had been agreed by the participants. A selectin f these phtgraphs is presented here tgether with cmments n hw the varius znes in the sectins were interpreted. Prcedure fr Age Interpretatin Althugh mre sphisticated methds f preparing and examining tlith sectins fr age determinatin are available, the prcedure utlined belw has been fund acceptable in sme labratries. Preparatin f tliths The tliths after remval frm the fish are washed and then stred in a dry cnditin. Preparatry t examinatin, the tlith is brken acrss the sulcus acusticus with the aid f a scalpel, and the halves partially embedded in mdeling clay with the brken surfaces upward. Care shuld be taken in making the fracture, because, if the cut is nt made crrectly, the aspect f the annuli n the cut surface changes and an errr in ageing may result. Micrscpic examinatin The fractured surfaces f the tlith halves are examined by transmitted light under lw pwer (abut 1 x ) f a bincular micrscpe. The angle and amunt f illuminatin shuld be such that the relevant details can be distinguished. A scalpel blade held n the distal edge f the cut surface will shade the surface frm direct illuminatin. A drp f alchl n the cut surface will enhance the examinatin. The accmpanying phtgraphs were made frm tliths prepared by this simple methd. Interpretatin f age Opaque and hyaline znes are distinguishable in the tlith sectin. The paque znes are assciated with perids f fast grwth and are usually frmed during the summer and autumn. yaline znes represent perids f slw grwth and are usually frmed in the winter and spring. Therefre, ver a perid f 1 year, an paque zne and a hyaline zne wuld be expected t be added t thse already existing in the tlith. In practical terms, if all hyaline znes are annuli, the number f hyaline znes can be cnsidered as the age if the fish's assumed birthday has been reached. The fish's birthday. Because the actual birthday f the sampled specimen is unknwn, it is usual t assume an arbitrary birthdate and January 1 is cnvenient fr this purpse. The relatinship between the

2 Sci. Cun. Studies, N.1, 191 number f paque and hyaline znes and the fish's age at capture might be as fllws: N. f N. f paque hyaline Edge f Fish age Perid znes znes tlith (years) Jan-Apr yaline May-Aug Opaque Sep -Dec Narrw hyaline It is usual that tliths f yunger cd shw new paque grwth earlier in the summer and new hyaline grwth earlier in the autumn than lder cd. As nted in the abve example, it is in the autumn that difficulties arise in determining the age frm the number f hyaline znes. If the hyaline zne at the edge f the tlith was frmed in the previus winter, the age is equal t the number f hyaline znes. If, n the ther hand, the hyaline zne at the edge was recently frmed, it is nt cunted in determining the age. Checks and splits. Because f variable envirnmental and feeding cnditins, narrw hyaline znes may smetimes be frmed during the perid when grwth is nrmally fast and narrw paque znes may ccur in the winter when grwth is expected t be negligible. Cnsequently, nt all hyaline znes are annuli. A check is a hyaline zne in the middle f an paque zne and shuld nt be interpreted as a winter zne r annulus. A split is a cmpsite hyaline zne cnsisting f tw r mre clsely spaced hyaline znes f the same pattern which cmprise ne annulus. Als, sme hyaline znes may nt be well defined, and it may be difficult t determine if these are in fact annuli. Annuli patterns. Althugh the past histry f the fish frm which the tlith was taken is unknwn, it is reasnable t assume that its grwth ccurred in accrd with the vn Bertalanffy grwth pattern, i.e. a series f decreasing increments fllwing the first r years f rapid grwth. Therefre, if a relatinship between fish length and tlith radius is accepted, the pattern f the annuli alng a line frm the center f the tlith sectin tward the periphery shuld appear as a series f prgressively declining increments. If such a pattern is clearly discernable, the prblem f distinguishing checks and splits will be very much lessened. Use f tlith size and fish length. In the east Newfundland-suth Labradr cd stck (Div. J, K and L), the size ranges by age-grup taken in the cmmerical fishery verlap t such an extent (Appendix Table 1) that the use f fish length r tlith radius is nt a reliable indicatr f age, and, if used in cnjunctin with an interpretatin f the tlith pattern, may in fact be a surce f bias. Use f cnstant magnificatin. Despite the verlap in the tlith radius fr different age-grups f cd, it is clear that, n the average, the tlith radius increases with age. The fact that many expert tlith readers nt nly maintain a fixed magnificatin but als prefer a particular micrscpe when interpreting tlith sectins implies that tlith size assists the reader in determining the age. Age validatin. While the ageing f juvenile cd may be validated by the ccurrence f relatively distinct mdal grups in length frequencies frm catches in small-mesh research trawls, the methd cannt be used directly fr lder cd f cmmercial sizes. wever, the experience gained in bserving the annuli patterns in tliths f juvenile cd (ages 1-) is extremely useful in interpreting the patterns in tliths f lder fish. The ccurrence f dminant year-classes and the similarity f grwth patterns frm the results f ageing wuld lead t cnfidence that tliths are being cnsistently interpreted. Cmparisn f age cmpsitin and mean length-at-age data reprted by different labratries fr the same cd stck wuld indicate whether there were substantial discrepancies r biases. The ccasinal exchange f tliths within and between labratries may act as a safeguard against vercnfidence in age determinatin. Selected Phtgraphs f Otlith Sectins Details relevant t interpretatin f the 9 tlith sectins illustrated in the Appendix are given in Table 1. The first 1 speciments with numbers prefaced by J r K were cllected in these divisins during Nvember and Decem ber 19, and each wu Id have been aged 1 year lder if it had been taken n r after 1 January 19. The specimens with numbers prefaced by J, K and L were cllected in these divisins in February 19, April 19 and June 19 respectively. Examples f symbls used t designate the type f zne at the edge f the tlith and the varius checks and splits are as fllws: = hyaline zne = paque zne D, = diffuse first hyaline zne W = wide secnd hyaline zne C = check in secnd paque zne = secnd hyaline zne split. The illustrated sectins, phtgraphed under cnstant magnificatin, are cnsidered typical f annuli patterns, at leastfr age-grups t, fu nd in tliths f cd frm the stck cmplex ff eastern Newfundland and suthern Labradr, and are presented with a view t their ptential use as aids t cd tlith interpretatin by ther wrkers.

3 WELLS: Cd Age Determinatins frm Otliths 9 References ICNAF. 19. Age-length keys fr cd. ICNAF Sampl. Yearb., Vl. 1: ICNAF. MS 19. Preliminary reprt f Ageing Wrkshp n Cd held at St. Jhn's, Canada, 1 January- February 19 (R. Wells, cnvener). ICNAF Sum. Dc.., N. /1, Serial N. 0. LOPEZ-VEIGA, E. C., R. WELLS, and V. M. ODDER (Eds.) 19. Reprt f Ageing Wrkshp n Cd held at Vig, Spain, Octber 19. ICNAF Sel. Papers, N.: 1-0. APPENDIX APPENDIX TABLE 1. Ages and cmments relative t the phtgraphs f tliths n the fllwing pages. Pht. N.. Spec. N. 1 K-1 K- K- K- K-9 K-1 J- K-1 9 K J- 11 J- 1 J-9 1 J-10 1 J-1 1 K- 1 J-0 1 K- 1 J- 19 L-1 0 K-1 Fish length (cm) Fish age (yr) 1 Edge f tlith Cmments s, S, 0, S, S,,,,,, C"S, W,, Pht. N. Spec. N. 9 J- 0 K- 1 L- K- L- L- K- L- L- L- 9 J-1 0 J-0 1 J- J- K-1 K-0 J-B J-9 K- K-1 Fish length (cm) Fish age (yr) Edge f tlith Cmments C" c.. S,,, S" S" S,,,, " " S" ",, C" " C C" " " S,,, s,,,, W, C" S", C" S" " C" Ss W" S" S W" s" S, W", " S, 1 K- L- L-1 J W"S,,,, C" S", 9 K- 0 K- 1 L- L-9 1 " S" S, " S,. C J- J- L-11 K- 9 K- 0 L-1 1 L-1 L- K- K-9 L-9 K-10 K-1 L-0 9 L- 0 L-0 1 K- J- J- J- J-9 K-1 K-1 K ,,, S,,,, S,,, S 0" C S" s", S, S" cut ff center S" W" S C, S,, C" " S, " S,,,, C. C" C" S" S, s" C, S " C S, S L-9 J-1 J-0 J- J- J-9 9 J- 0 K- 1 K- K-9 L-1 L-9 J-1 K- J- L-90 9 L L-9 91 L J- 9 K- 9 K- 9 K " " S, C" S, S " S S, C C" C" " C., S " C" " S " Ss,,, S C" C" S" " C" S, S,,,, S. S, S. S C" C" C, S" " S" S W" S",, S" ", Ss C" C, C

4 0 Sc:. Sci. Cu~ Gun.. Studies, N. 1, j 191 igs1

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Holger H. Hansen Greenland Fisheries and Environmental Research Institute Tagensvej 135, Copenhagen N, DK-2200, Denmark. Abstract

Holger H. Hansen Greenland Fisheries and Environmental Research Institute Tagensvej 135, Copenhagen N, DK-2200, Denmark. Abstract NAFO Sci. Cun. Studies, : 7- han Hlger H. Hansen Greenland Fisheries and Envirnmental Research Institute Tagensvej, Cpenhagen N, DK-00, Denmark Abstract Analysis f sampling data fr Atlantic cd (Gadus mrhua)